Jinja2 Exception Handling

While jinja2 does not have a way to handle this by default, there is a workaround.

Since try is not supported in the template language, we need a helper function defined in Python, like this:

def handle_catch(caller, on_exception):
    try:
         return caller()
    except:
         return on_exception

This method is injected into the template engine, either via the Environment.globals or when calling the render method. In this example it is injected via the render method.

my_template.render(handle_catch=handle_catch)

In the template itself it is then possible to define a macro:

{% macro catch(on_exception) %}
    {{ handle_catch(caller, on_exception) }}
{% endmacro %}

And this can then be used as:

{% for item in items %}
   {% call catch('') %}
       {{ item | custom_urlencode_filter }}
   {% endcall %}
{% endfor %}

Notes:

• The caller method is provided by the jinja2, and this is a function that renders the code between {% call ... %} and {% endcall %}
• on_exception can be used to provide an alternative text in case of exceptions, but in this case we just use an empty string.

{% for item in items %}
   {{ item | custom_urlencode_filter }}
{% endfor %}

Then in whatever file you have setting up your jinja2 environment

def custom_urlencode_filter(value):
    try:
        return urlencode(value)
    except:
        # handle the exception


environment.filters['custom_urlencode_filter'] = custom_urlencode_filter

More on custom jinja2 filters